1. Field of the Invention
The present invention generally relates to an expected value generation unit used by a data reproduction apparatus, such as an optical disk apparatus and a magnetic disk apparatus, and specifically relates to the expected value generation unit that generates an expected value used when reproducing data according to maximum likelihood decoding algorithms, such as a Viterbi decoding algorithm.
Further, the present invention relates to a data reproduction apparatus that performs reproduction of data using the expected value generated by the expected value generation unit.
2. Description of the Related Art
An optical recording medium, such as an optical disk, and an optical magnetic disk, of an optical disk apparatus is widely used in various fields, such as an auxiliary memory unit of a computer, due to large capacity, portability, high reliability, and so on. As for an optical disk apparatus, recording and reproduction of data with a higher precision are required as the recording density is increasing.
As for recording and reproducing data with a high precision with the optical disk recording medium, a PRML technique has been proposed, wherein data modulation recording corresponding to partial-response (PR) wave is performed, and reproducing most probable data are performed, employing a maximum likelihood decoding (ML) using sampled values obtained by sampling a signal to be reproduced at a predetermined sampling frequency, the signal being read from the optical disk recording medium.
An example of a data reproduction apparatus that uses the ML technique in reproducing original signals from the optical disk recording medium to which data are recorded using the PR technique, is configured as shown in FIG. 1.
As shown in FIG. 1, the data reproduction apparatus includes an optical head 20 that optically scans an magneto optical disk 200 to which data are recorded by the PR technique, reads the data, and outputs a signal of the read data, an amplifier 21, a low pass filter (henceforth LPF) 22, an analog to digital converter (henceforth ADC) 23, a digital equalizer (henceforth EQ) 24, a synchronous clock generation unit 25, and a maximum likelihood decoding unit 100. The signal read from the optical head 20 is supplied to the ADC 23 through the amplifier 21 and the low pass filter 22, as a signal to be reproduced. The ADC 23 samples the signal to be reproduced in synchronization with the synchronous clock supplied from the synchronous clock generation unit 25, and outputs sampled values sequentially. The EQ 24 operates in synchronization with the synchronous clock, and applies waveform equalization processing of a PR wave to the sampled values from the ADC 23.
The sampled values that are waveform-equalized by the EQ 24 are supplied to the maximum likelihood decoding unit 100 sequentially. The maximum likelihood decoding unit 100 includes, for example, a Viterbi decoder, and reproduces most probable data according to the Viterbi decoding (maximum likelihood decoding) algorithm from the sampled values supplied sequentially, and outputs the data sequentially.
The maximum likelihood decoding unit 100 that processes according to the Viterbi decoding algorithm is configured as shown in FIG. 2.
In FIG. 2, the maximum likelihood decoding unit 100 includes a branch metric calculation unit 10 (henceforth, simply called BM), an addition-comparison-selection unit 11 (Add-Compare-Select, henceforth simply ACS), a path metric memory unit 12 (henceforth, PMM), and a path memory unit 13 (henceforth, PM).
The BM 10 calculates a branch metric value (henceforth BM value) based on a difference between a sampled value yt supplied sequentially and an expected value. The expected value is a value based on a partial response wave used when recording, and is the value that a reproduced signal should take. The BM value is calculated for every expected value, when a sampled value yt is supplied to the BM 10.
The ACS 11 adds the BM value of a current clock cycle to a path metric value (henceforth, PM value) of a previous clock cycle, stored in the PMM 12 (Add), and compares every two PM values after the addition (Compare). Then, the ACS 11 selects a smaller value of the two PM values compared, as a new PM value (Select), and stores the selected PM value in the PMM 12. Consequently, the PM value turns into an accumulated sum of BM values through processing such as above. Selecting a certain PM value as mentioned above is equivalent to selecting a path of a state transition. That is, the ACS 11 always selects a path of a state transition the PM value of which is the minimum.
Data (binary data) equivalent to the path selected as mentioned above are supplied to the PM 13 from the ACS 11. The PM 13 shifts the data corresponding to each selected path one by one, and screens data corresponding to paths that are not to be selected, based on continuity of the state transition in the shifting process. Then, the PM 13 outputs data corresponding to surviving paths as detected data.
As mentioned above, the maximum likelihood decoding unit 100 performs the likeliest reproduction of the data, based on the sampled value yt sequentially input and the expected value based on the PR wave. The expected value corresponds to a sampled value that should duly be acquired from the PR wave. If the wave of a signal to be reproduced is correctly in agreement with the PR wave, accurate reproduction of data is possible, even if the expected value is a fixed value. However, if a fixed expected value is used when distortion etc. is present in the signal to be reproduced, accurate reproduction of data are not expectable.
A technique is proposed, whereby an expected value is decided from a distribution state of the sampled values expressed on a histogram. The histogram is generated from the sampled values of the signal to be reproduced. For example, in the case of a PR(11) wave that has three expected values, the histogram of the sampled values of the signal to be reproduced shows three peaks, as shown in FIG. 4 (b). Sampled values corresponding to the three peaks are used as the expected values of the PR(11) wave.
However, if an offset arises in the signal to be reproduced, for example, due to envelope fluctuation, and the like, as shown in FIG. 3, variations in the sampled values become large, and peaks are not clear in the histogram as shown in FIG. 4(a). For this reason, reliability of the expected value determined from such a histogram is low, and an ability to reproduce correct data declines.
It is a general object of the present invention to provide an expected value generating unit and a data reproduction apparatus that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by the expected value generating unit and the data reproduction apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a first objective of the present invention is to provide an expected value generating unit that is capable of providing a correct expected value, even if there occurs an offset in the signal to reproduce.
A second objective of the present invention is to provide a data reproduction apparatus that uses the expected value generation unit.
In order to achieve the first objective of the present invention, the expected value generation unit used by the data reproduction apparatus includes an offset detection unit that detects an offset amount of a signal to be reproduced, an offset cancellation unit that removes the offset detected by the offset detection unit from sampled values of the signal to be reproduced, and a histogram generation unit that generates a histogram of the sampled values from which the offset has been removed by the offset cancellation unit. In this manner, the expected value is determined based on the histogram of the sampled values, the histogram being generated by the histogram generation unit.
In the expected value generation unit, if an offset is detected in a signal to be reproduced, the offset is removed from the sampled values of the signal to be reproduced. Then, the histogram of the sampled values from which the offset has been removed is generated, and an expected value is determined from the generated histogram. Thus, since the sampled values from which the offset was removed become close to the sampled value from an original signal, there appear peaks that correspond to the original signal wave, in the histogram generated from the sampled value.
Further, in order to obtain a correct histogram even if an amount of the offset changes suddenly, the expected value generation unit includes a unit that detects a period during which the offset amount is expected to changes suddenly, and a control unit that excludes sampled values during the period.
In the expected value generation unit configured as above, a sudden change of the offset amount, which deteriorates stability of the offset amount detection, is removed from the sampled values from which the histogram is generated.
The second objective of the present invention is achieved by configuring a data reproduction apparatus employing the expected value generation unit mentioned above.